For example, Patent Document 1 listed below discloses the following technique as a conventional technique for producing partially reduced iron by reducing agglomerates containing iron oxide. Agglomerate-like reduced iron is stacked on a grate of a horizontally-circulating grate reduction furnace, pellets are stacked on the reduced iron and are ignited, and pellets are further stacked after the ignition. Meanwhile, an oxygen-containing gas is made to flow from below to above the agglomerate-like reduced iron and the oxygen concentration of the oxygen-containing gas is reduced stepwise along with the movement of the pellets to the downstream side. The pellets are thereby reduced sequentially from a lower layer to an upper layer of the pellet bed and the partially reduced iron is produced.
Moreover, Patent Document 2 listed below discloses the following technique. Pellets (raw material) formed by mixing and pelletizing powders of iron ore, coal, limestone, and the like are supplied to the reduction furnace and are exposed to a high-temperature atmosphere. Meanwhile, the oxygen concentration of a gas to be supplied into the furnace is adjusted based on a detected temperature inside the furnace to control the temperature inside the furnace. The pellets are thereby reduced and the partially reduced iron is produced.